Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly.
FIG. 1 schematically illustrates an existing web-format planarizing machine 10 for planarizing a substrate assembly 12. The planarizing machine 10 has a support table 14 with a top panel 16 at a workstation where an operative portion (A) of a polishing pad 40 is positioned. The top panel 16 is generally a rigid plate to provide a flat, solid surface to which a particular section of the polishing pad 40 may be secured during planarization.
The planarizing machine 10 also has a plurality of rollers to guide, position and hold the polishing pad 40 over the top panel 16. The rollers include a supply roller 20, first and second idler rollers 21a and 21b, first and second guide rollers 22a and 22b, and a take-up roller 23. The supply roller 20 carries an unused or preoperative portion of the polishing pad 40, and the take-up roller 23 carries a used or postoperative portion of the polishing pad 40. Additionally, the first idler roller 21a and the first guide roller 22a stretch the polishing pad 40 over the top panel 16 to hold the polishing pad 40 stationary during operation. A motor (not shown) drives at least one of the supply roller 20 and the take-up roller 23 to sequentially advance the polishing pad 40 across the top panel 16. As such, clean preoperative sections of the polishing pad 40 may be quickly substituted for used sections to provide a consistent surface for planarizing and/or cleaning the substrate assembly 12.
The web-format planarizing machine 10 also has a carrier assembly 30 that controls and protects the substrate assembly 12 during planarization. The carrier assembly 30 generally has a substrate holder 32 to pick up, hold and release the substrate assembly 12 at appropriate stages of the planarizing cycle. A plurality of nozzles 33 attached to the substrate holder 32 dispense a planarizing solution 44 onto a planarizing surface 42 of the polishing pad 40. The carrier assembly 30 also generally has a support gantry 34 carrying a drive assembly 35 that translates along the gantry 34. The drive assembly 35 generally has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the substrate holder 32 via another shaft 39 such that the drive assembly 35 orbits the substrate holder 32 about an axis B--B offset from a center point C--C the substrate assembly 12.
The polishing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate assembly 12. The web-format planarizing machine 10 typically uses a fixed-abrasive polishing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution is generally a "clean solution" without abrasive particles because the abrasive particles are fixedly distributed across the planarizing surface 42 of the polishing pad 40. In other applications, the polishing pad 40 may be a nonabrasive pad composed of a polymeric material (e.g., polyurethane), a resin, or other suitable materials without abrasive particles. The planarizing solutions 44 used with nonabrasive polishing pads are typically CMP slurries with abrasive particles and chemicals to remove material from a substrate.
To planarize the substrate assembly 12 with the planarizing machine 10, the carrier assembly 30 presses the substrate assembly 12 against the planarizing surface 42 of the polishing pad 40 in the presence of the planarizing solution 44. The drive assembly 35 then orbits the substrate holder 32 about the offset axis B--B to translate the substrate assembly 12 across the planarizing surface 42. As a result, the abrasive particles and/or the chemicals in the planarizing medium remove material from the surface of the substrate assembly 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate assembly 12 to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other components, many substrate assemblies develop large "step heights" that create a highly topographic surface across the substrate assembly 12. To enable the fabrication of integrated circuits with high densities of components, it is necessary to produce a highly planar substrate surface at several stages of processing the substrate assembly 12 because nonplanar substrate surfaces significantly increase the difficulty of forming submicron features. For example, it is difficult to accurately focus photopatterns to within tolerances approaching 0.1 .mu.m on nonplanar substrate surfaces because submicron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
In the competitive semiconductor industry, it is also highly desirable to have a high yield in CMP processes by quickly producing a uniformly planar surface at a desired endpoint on a substrate assembly 12. For example, when a conductive layer on a substrate assembly 12 is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate assembly 12 over a dielectric layer. Additionally, when a substrate assembly 12 is over planarized, components below the desired endpoint may be damaged or completely destroyed. Thus, to provide a high yield of operable microelectronic devices, CMP processing should quickly remove material until the desired endpoint is reached.
One technique to improve the performance of CMP processing is to use fixed-abrasive pads (FAPs) with a clean planarizing solution instead of nonabrasive pads with abrasive slurries. One problem with abrasive slurries is that the slurry may not uniformly contact the face of a substrate assembly 12 because the leading edge of the substrate assembly 12 wipes the slurry off of the pad 40. As a result, more abrasive particles generally contact the edge of the substrate 12 assembly than the center, causing a center-to-edge planarizing profile. FAPs seek to resolve this problem by fixedly attaching the abrasive particles to the pad in a desired distribution. By fixing the abrasive particles to the pad instead of suspending the abrasive particles in the slurry, the center of the substrate assembly 12 contacts a large number of abrasive particles irrespective of the distribution of planarizing solution between the pad and the substrate assembly 12. Using FAPs, however, presents some drawbacks in CMP processing.
One drawback of existing FAPs is that the abrasive particles in the FAPs may not adequately planarize substrate assemblies with very small components (e.g., components with a dimension of 0.25 .mu.m or less). Existing FAPs are typically fabricated by covering a Mylar.RTM. or polyurethane backing film with a layer of resin and abrasive particles. The resin is then cured, and the layer of cured resin and abrasive particles may be textured. The particle size distribution of the abrasive particles in FAPs should: (1) be consistent from one pad to another to provide consistent planarizing results; and (2) have small particle sizes that are generally less than the critical dimension of the smallest components to avoid producing defects and to form a very smooth surface on the substrate assembly. The particle size distribution in FAPs, however, may not be small enough to planarize very small components because individual abrasive particles may agglomerate into larger abrasive elements that have a plurality of individual particles. For example, FAPs may have abrasive particles with individual particle sizes of approximately 10-250 nm, but the individual particles may agglomerate together to form relatively large abrasive elements in the resin having a size distribution from 0.2-1.5 .mu.m. The formation of such large abrasive elements alters the consistency of the FAPs because the extent that the particles agglomerate varies from one pad to another, or even within a single pad. Additionally, large abrasive elements may scratch the substrate assembly and produce defects, or they may damage very small components of the integrated circuitry on a substrate assembly. Thus, the agglomeration of abrasive particles into larger abrasive elements is a serious problem for fabricating very small electronic components with FAPs.
Another drawback of FAPs is that it is difficult to obtain the desired distribution of abrasive particles in the resin even when the individual abrasive particles do not form a significant number of larger abrasive elements. For example, it is generally difficult to control the distribution of the abrasive particles in the resin because the resin typically has a relatively high viscosity that inhibits uniform mixing of the abrasive particles. One particularly difficult application is producing FAPs with ceria abrasive particles because it is difficult to manufacture small ceria particles and it is difficult to uniformly mix ceria particles in a liquid. Thus, even if the abrasive particles do not agglomerate extensively, it is still difficult to obtain a desired distribution of abrasive particles at the planarizing surface of an FAP.
Still another concern of using FAPs is that these pads are relatively expensive and may wear out rather quickly. FAPs are relatively expensive because of the difficulties in obtaining sufficiently small particle sizes and a desired distribution of the abrasive particles, as explained above. Moreover, FAPs are subject to wear because the substrate assembly rubs against the resin at the planarizing surface causing the resin to wear down. As a result, some of the abrasive particles may detach from the resin and cause defects, or the abrasiveness of the pad may be sufficiently altered to produce inconsistent planarizing results. Therefore, using FAPs may increase the costs of planarizing microelectronic-device substrate assemblies.